Four Key Features to look for in a GaN-Ready Magnetics Supplier
The dramatic rise in the popularity of Galium Nitride (GaN) switching technology is due to the very significant benefits it provides. Highly efficient GaN devices offer high dielectric strength, high operating temperatures, high current density, high-speed switching and exceptional carrier mobility.
By Welly Chou, Design Engineering Manager, Precision Inc.
Several of these benefits - most notably high switching frequencies and high operating temperatures - can have a significant impact on the performance of a system´s magnetic components. To ensure optimal performance of magnetic elements, it is essential to locate a supplier that specializes in GaN-ready magnetics. Failure to do so can result in the magnetics becoming a bottleneck for system efficiency.
There are four key features to look for in a quality GaN-ready magnetics supplier:
Core Materials Selection
The high switching frequencies and high operating temperatures made possible by GaN technology bring key benefits to design engineers. They also provide a significant challenge in core material selection for magnetic components as the performance of these components varies significantly based on both factors.
Be sure to choose a magnetics component supplier who understands how to select the optimal core material for your unique GaN-based technology. To do this, make sure that a potential supplier has:
Expertise In A Wide Range of Core Materials
A quality supplier will have in-depth knowledge of the performance characteristics of at least 50 different core materials including how they perform at varying operating temperatures and switching frequencies.
Expertise in Core Material Performance and Switching Frequency
Ask specific questions about how the performance of any selected core material is impacted by switching frequency. The same material will perform very differently at different frequencies. Figure 1 shows that Material A exhibits five times more core loss when operating at 300kHz compared to 100kHz switching frequencies (assuming a common 0.1T peak flux density).
Another way to interpret such difference is that in order to achieve the same core loss (100mW/cm3) at both 100kHz and 300kHz, the 300kHz operating peak flux density must be de-rated by 44% from that at 100kHz. (peak flux density of 0.053T @ 300kHz vs. 0.095T @ 100kHz).
A common misconception is that as operating frequency increases (3X in this case from 100kHz to 300kHz), the number of turns can be reduced by the same ratio (3X). However, the gain in turns reduction is not directly proportional to the increase in switching frequency. Taking the above peak flux density de-rating into consideration, in order to achieve the same core loss, the turns reduction is 1.7X instead of 3X.
Better core material selection can reduce loss variability with switching frequencies. As demonstrated in Figure 1, Material B is a better choice for 300kHz switching frequencies as the required peak flux density is 0.07T in order to achieve the same 100mW/cm3 core loss. Being able to operate at a higher peak flux density allows the turns reduction to be 2.21X. This is a 32% improvement when compared to Material A (1.7 X turn reduction). This will help to further reduce conduction loss and winding capacitance.
Expertise in Core Material Performance and Operating Temperature
A quality supplier will also be able to provide precise information about the best core material selection for your application’s unique performance environment. Losses vary by as much as 50% at various temperatures with a given core material. Figure 2 shows the core loss variance versus temperature for Materials A and B (at 100kHz and 0.1T).
Material A exhibits its lowest core loss around 100°C. However, core loss can vary greatly with operating temperature. For example, with Material A, core loss is 50% higher at 40°C and 10% higher at 120°C than was the case at 100°C. On the other hand, the core loss of Material B remains a lot more stable over temperature with loss variance between 40°C and 100°C within 10% from its minima.
The important role of operating temperature is often overlooked in an industry where it is common practice to chart core loss curves at a fixed temperature (most often the minima). Selecting the proper core material for your application´s operating temperature can have a significant impact on efficiency. In this instance, selecting Material B over Material A might make the difference between an 80 Plus Platinum and 80 Plus Titanium efficiency certification.
Both leakage inductance and capacitance contribute to switching losses. Leakage inductance has been known to create voltage spikes during switching. Depending on their unique application, design engineers will want to minimize or maintain leakage inductance. In nearly all cases design engineers will be looking to minimize capacitance. To achieve precise parasitic management, identify a GaN-ready magnetics supplier who uses both fi nite element analysis and a variety of winding configurations.
3D Finite Element Analysis
Three Dimensional Finite Element Analysis is used to analyze magnetic flux distribution and leakage inductance by analyzing losses due to skin and proximity effects. Electromagnetic properties are modeled and investigated with advanced Maxwell 3D simulation software from Ansys. In addition to ensuring optimal performance, finite element analysis speeds time to market by ensuring performance is optimized from the beginning, eliminating the need for design reiterations.
Advanced Winding Configurations
Quality GaN-ready magnetic suppliers will also have advanced winding configuration capabilities. The same magnetic component with the same bobbin, number of turns and package style can have significantly different capacitance dependent on the winding configuration. As you can see in Figure 3, capacitance can vary as much as 75% dependant solely on winding configuration.
Be sure any potential GaN-ready magnetic supplier has the advanced winding (both bobbin wound and toroid wound) expertise to minimize parasitics in each unique application.
Parasitic Management Results
Using a combination of Finite Element Analysis and advanced winding configurations, quality GaN-ready magnetic suppliers can precisely manage both leakage inductance and capacitance parasitics. For design engineers looking to maintain leakage inductance, single section bobbins can be created that provide three to five times more leakage inductance than a traditional bobbin. This is done using a single section bobbin where the primary and secondary windings are concentrically wound.
For those looking to minimize leakage inductance, quality suppliers should be able to design a solution where inductance accounts for less than one percent of total part inductance. Additionally, the combination of Finite Element Analysis and advanced winding configurations will allow GaN-ready magnetic experts to provide as much as five times lower capacitance than traditional magnetic designs.
It is important to ensure that your GaN-ready magnetics supplier goes beyond “theoretical” simulated Finite Element Analysis results to create validated DOE (Design of Experiment) with physically constructed tests.
Extensive Litz Wire Selection
Different switching frequencies require different strand sizes of Litz wire for optimal performance. This is due to both Skin and Proximity Effects.
As switching frequency increases, current tends to travel on the outside of the conductor, increasing its AC resistance. This phenomenon is called Skin Effect. The skin depth of a conductor at a given frequency is defined as the penetration distance from the surface towards the center of the conductor. Figure 4 demonstrates skin depth vs. frequency.
Single-stranded wire can be chosen based on skin depth performance to optimize its AC/DC resistance ratio. The goal is to reach the unity condition wherein the conductor is fully utilized for carrying high frequency current without any wasted non-current carrying space in the center. It has been proven that such unity condition is achieved by selecting wire with twice the diameter of the skin depth. Figure 5 shows the skin depth vs. single strand wire AWG, achieving the unity AC/DC ratio.
Skin Effect, however, is only a part of AC resistance that hinders overall efficiency. Another critical contributor to AC resistance is the Proximity Effect which is the current redistribution within a conductor caused by the current flowing in an adjacent conductor. Depending on the design, Proximity Effect can increase the AC resistance of a transformer or inductor by 10 to100 times more than Skin Effect can.
Litz Wire For Optimal Performance
Litz wire has traditionally been used to combat both Skin and Proximity Effects. Litz wire is made with a bundle of smaller strand size wire. Figure 6 illustrates commonly used Litz wire strand sizes vs. switching frequency.
It is worthwhile to note that, for the same frequency, the strand size in Figure 6 is significantly smaller than the single strand wire AWG in Figure 5 due to Proximity Effect. Such difference further outlines the significance of proper Litz wire selection. Selecting wire strand size incorrectly can prove to be costly to your system efficiency and component temperature.
Leading GaN Technology Partners
Finally, a quality GaN-ready magnetics supplier will be able to provide demonstrated results of their partnerships with leading GaN technology suppliers like International Rectifier and Transphorm. They will be able to provide firm performance data, based on full demo boards built with industry-leading GaN switch suppliers.
In summary, GaN switches provide an exciting number of key benefits for design engineers. These same benefits can also create challenges in the design of magnetic components. Be sure that any potential magnetic component supplier has advanced expertise in core material selection, parasitic management and an extensive selection of Litz wire options as well as proven partnerships with industry-leading GaN technology suppliers in order to ensure that your magnetics are optimized and helping you achieve a highly efficient system.